This invention relates to an apparatus for displaying a graph on a display screen (for example, the phosphor screen of a cathode-ray tube), and more particularly to a graph display apparatus in which means are provided so that the accuracy or delicacy of display (resolution of display) can be varied as required.
Display apparatus using raster scanned cathode-ray tubes for display are generally classified into a graphic display apparatus capable of relatively freely displaying a graph, a drawing, etc. on its display screen, and a character display apparatus capable of displaying characters, simple symbols, etc. on its display screen according to a combination of predetermined patterns.
The graphic display apparatus includes a memory which stores, for example, a data for unblanking selected ones of all the dots on the display screen (the dot being the minimum unit of display element on the display screen). The memory also stores data instructing the state of display by the dots, for example, the color of the unblanking or displaying dots. The data stored in the memory are read out to be supplied to the display unit including the cathode-ray tube in synchronous relation with the scanning of dots with the electron beam so as to selectively unblank the dots in the instructed positions in the desired color thereby displaying a desired graph or pattern on the display screen. Such a graphic display apparatus can display many graphs or patterns on its display screen at the same time since all the data corresponding to the individual dots on the display screen are stored in the memory. However, the capacity of the memory in this graphic display apparatus is very large due to the fact that all the data corresponding to the individual dots on the display screen must be stored in the memory. For instance, a memory capacity of 1,024.times.1,024=1,048,576 bits is required in the case of display by a cathode-ray tube (CRT) having a display area of 1,024 bits in the horizontal direction and 1,024 bits in the vertical direction of the display screen. The above figure represents the case of black-and-white display. In the case of color display with seven colors, 3-bit color information is required for each individual dot, and therefore, the required memory capacity is three times as large as that of the memory capacity required for the black-and-white display. The increase in the memory capacity results not only in an increase in the scale of the memory, but also in an extended length of time required for the writing and reading of dot information, and this leads to a delayed response of the display apparatus. This delayed response is objectionable in a process control or like system in which the state of process control is continuously monitored while viewing the display screen of such a display apparatus.
Thus, the commonly known graphic display apparatus involves the problem of complex circuitry and large scale and the problem of delayed response.
In contrast to such a graphic display apparatus, an apparatus is known wherein one graph display unit can operate at a high response rate and can realize the desired accurate graph display in spite of its simple structure although it can only display a single graph on the display screen of a CRT. Such a display unit is disclosed in, for example, Japanese Patent Publication No. 51-48862. The basic principle of the disclosed display unit will now be described. According to the basic principle of the display unit displaying a single graph, the time axis extending in the horizontal direction of the display screen of the CRT is equally divided into n parts, and T/n, obtained by dividing the scanning time T of one raster by the number n, is called one time point. This one time point is used as a unit for the plotting of graph display, and a memory is prepared to cover these time points so as to store the process variable of the inputs to be displayed at the individual time points. The term "process variable to be displayed" is used herein to designate the so-called vertical height relative to the horizontal direction of the display screen of the CRT. This height can be represented by the raster number of the scanning line scanning horizontally across the display screen of the CRT of the so-called raster scanning type. Thus, the raster numbers corresponding to the process variable to be displayed at the individual time points are stored in the memory, and during the scanning, the contents of the memory at the individual time points are sequentially read out. When the output of the memory corresponding to a specific raster number at a specific time point coincides with the output of a raster counter representing the present scanning position of the raster, corresponding dots on the display screen being scanned by the raster are caused to unblank at the specific time point. In this manner, a graph can be displayed on the CRT display screen by storing in a memory the raster numbers (corresponding to the process variable to be displayed) at the individual horizontally divided time points, sequentially reading out the contents of the memory during the scanning of the display screen by the raster, and causing the corresponding dots to unblank when coincidence is reached between the output of the memory and the output of the raster counter. According to this method, the memory capacity can be greatly reduced to quicken the response compared with the aforementioned graphic display apparatus, due to the fact that a memory having the same capacity as the number of horizontal time points is merely required. However, such a display unit is capable of only displaying a single graph. Therefore, two or more of such display units are prepared, and the outputs of these display units are applied to the CRT display unit through an OR circuit so as to display two or more graphs.
In the proposed display unit, the horizontal axis or time axis extending in the scanning direction of the raster is equally divided into n parts to provide n time points, and the process variable (actually, the raster numbers corresponding to the process variable) at these n time points are stored in the memory for displaying a graph. According to such an arrangement, however, a delicate or accurate graph display cannot be expected since only one information element is available for each time point. In other words, the graph cannot be displayed in the form of a smooth and continuous curve, and such a stepped display is relatively hard to be readily recognized by the eye.